286 
MR. C. T. R. WILSON ON CONDENSATION OF WATER VAPOUR 
point the tube was \iewed, out of the direct line of the incident light. The particles 
are then evidently small enough to scatter the red light less than the blue. 
Meteorological Applications. 
The question of the degree of supersaturation reached in these experiments is 
postponed till the results obtained with other gases have been given. In considering 
the meteorological applications we are directly concerned with the expansion required 
to cause condensation in air originally saturated. For adiabatic expansion to result 
in condensation in saturated air free from all foreign nuclei, we have seen that the 
final volume must exceed 1’252 times the inititd volume. 
To obtain the corresponding ratio between the final and initial pressures we have 
= IW. 
or 
ih ^ A'l Y. 
Ih V2/ 
where y is the ratio of the specific heat of air at constant pressure to that at 
constant volume. The difference in y for dry and moist air is small, and may here 
be neglected, y being therefore taken as equal to i'41. 
Now 
fl 
^3 
1 
1-252 
therefore 
Ih 
Ih 
1 
1.2521-ia 
728. 
Condensation must therefore take place in air free from foreign nuclei when the 
pressure is reduced adiabatically to 728 of the value which it has when the air 
is just saturated. The drops which are formed are, as we have seen, comparatively 
few. The fall of pressure required is sensibly the same for all values of the initial 
temperature between 28° and 15° C., and it is therefore probable that the result 
may be applied to considerably lower temperatures without any great error. 
It is natural to suppose that when there is an upward current of moist air, the 
foreign nuclei will be left behind through becoming loaded with the water which 
condenses on them, and that the air which rises above the lower cloud layer 
thus formed will be dust-free and supersaturated. 
It follows from the results of these experiments that condensation will again begin 
when the air reaches such a height that the pressure is reduced to about 73 of that 
at the upper surface of the lower cloud. 
It is not likely that the cloud-like condensation obtained with greater expansion 
has any meteorological significance. For it is unlikely that there can ever be such a 
sudden uprush of air as to enable any great degree of supersaturation to be main¬ 
tained when drops have already begun to form. 
